This application claims benefits of Japanese Application No. 2002-106378 filed in Japan on Apr. 9, 2002, the contents of which are incorporated herein by this reference.
The present invention relates generally to a zoom lens and an electronic imaging system using the same, and more particularly to an electronic imaging system such as a video camera or a digital camera, the depth dimension of which is diminished by providing some contrivances to an optical system portion such as a zoom lens.
In recent years, digital cameras (electronic cameras) have received attention as the coming generation of cameras, an alternative to silver-halide 35 mm-film (usually called Leica format) cameras. Currently available digital cameras are broken down into some categories in a wide range from the high-end type for commercial use to the portable low-end type.
In view of the category of the portable low-end type in particular, the primary object of the present invention is to provide the technology for implementing video or digital cameras whose depth dimension is reduced while high image quality is ensured, and which are easy to handle.
The gravest bottleneck in diminishing the depth dimension of cameras is the thickness of an optical system, especially a zoom lens system from the surface located nearest to its object side to an image pickup plane.
Recent technologies for slimming down cameras rely primarily on a collapsible lens mount that allows the optical system to be taken out of a camera body for phototaking and received therein for carrying. Typical examples of an optical system that can effectively be slimmed down while relying on the collapsible lens mount are disclosed in JP-A""s 11-194274, 11-287953 and 2000-9997. Each publication discloses an optical system comprising, in order from its object side, a first lens group having negative refracting power and a second lens group having positive refracting power, wherein both lens groups move during zooming.
According to one aspect of the present invention, there is provided a zoom lens, comprising, in order from its object side, a first lens group that remains fixed during zooming, a second lens group that has negative refracting power and moves during zooming, a third lens group that has positive refracting power and moves during zooming, and a fourth lens group that has positive refracting power and moves during zooming and focusing, characterized in that the first lens group comprises, in order from its object side, a negative meniscus lens component convex on its object side, a reflecting optical element for bending an optical path and a positive lens.
According to another aspect of the present invention, there is provided a zoom lens, comprising, in order from its object side, a first lens group that remains fixed during zooming, a second lens group that has negative refracting power and moves during zooming, a third lens group that has positive refracting power and moves during zooming, and a fourth lens group that has positive refracting power and moves during zooming and focusing, characterized in that the first lens group comprises a reflecting optical element for bending an optical path, and upon focusing on an infinite object point, the fourth lens group moves in a locus opposite to that of movement of the third lens group during zooming.
The advantages of, and the requirements, for the above arrangements used herein are now explained.
While relying upon the arrangement comprising, in order from its object side, the first lens group that remains fixed during zooming, the second lens group that has negative refracting power and moves during zooming, the third lens group that has positive refracting power and moves during zooming and the fourth lens group that has positive refracting power and moves during both zooming and focusing, the zoom lens of the present invention enables an associated camera to be immediately put into the ready state unlike a collapsible lens mount camera. To be favorable for water-proofing and dust-proofing purposes, the first lens group is designed to remain during zooming, and for considerably reducing the depth dimension of the camera, at least one reflecting optical element for bending an optical path is located in the first lens group nearest to the object side of the lens system.
However, the location of the optical path-bending reflecting optical element in the first lens group would give rise to the following two demerits.
A. The depth of an entrance pupil increases, leading unavoidably to an increase in the size of each lens element forming the first lens group that, by definition, has a large diameter.
B. The magnification of a combined system comprising the second or the third lens group that, by definition, has a zooming function and the subsequent lens group or groups is close to zero, and so the zoom ratio becomes low relative to the amount of zooming movement.
First of all, the condition necessary for bending is explained. Referring to a zoom type such as one intended herein, the location of the optical path-bending reflecting optical element in the first lens group necessarily makes the position of the entrance pupil likely to become deep, as in the case of JP-A 10-62687 or 11-258507, resulting in an increase in the size of each optical element that forms the first lens group. It is thus preferable that the first lens group comprises, in order from its object side, a negative meniscus lens component convex on its object side, a reflecting optical element for bending an optical path and a positive lens and satisfies the following conditions (1), (2), (3) and (4).
1.4 less than xe2x88x92f11/{square root over ( )}(fWxc2x7fT) less than 2.4xe2x80x83xe2x80x83(1)
1.2 less than f12/{square root over ( )}(fWxc2x7fT) less than 2.2xe2x80x83xe2x80x83(2)
0.8 less than d/L less than 2.0xe2x80x83xe2x80x83(3)
1.55 less than nPRIxe2x80x83xe2x80x83(4)
Here f11 is the focal length of the negative meniscus lens component in the first lens group, f12 is the focal length of the positive lens element in the first lens group, fW and fT are the focal lengths of the zoom lens at the wide-angle end and the telephoto end of the zoom lens, respectively, d is an air-based length from the image side-surface of the negative meniscus lens component to the object side-surface of the positive lens element in the first lens group, as measured on the optical axis of the zoom lens, L is the diagonal length of the (substantially rectangular) effective image pickup area of an electronic image pickup device, and NPRI is the d-line refractive index of the medium of a prism used as the optical path-bending reflecting optical element in the first lens group.
In order to locate the entrance pupil at a shallow position thereby enabling the optical path to be physically bent, it is preferable to increase the powers of the lens elements on both sides of the first lens group, as defined by conditions (1) and (2). As the upper limits of 2.4 and 2.2 to both conditions are exceeded, the entrance pupil remains at a deep position. Hence, when it is intended to ensure some angle of view, the diameter or size of each optical element forming the first lens group becomes too large to physically bend the optical path. As the lower limits of 1.4 and 1.2 are not reached, the magnification that the lens groups subsequent to the first lens group and designed to move for zooming can have becomes close to zero, offering problems such as an increase in the amount of zooming movement or a zoom ratio drop and, at the same time, rendering correction of off-axis aberrations such as distortion and chromatic aberrations difficult.
Condition (3) is provided to determine the length necessary for the location of the optical path-bending reflecting optical element, as measured along the optical axis of the zoom lens. Although the value of this condition should preferably be as small as possible, it is understood that as the lower limit of 0.8 thereto is not reached, a light beam contributing to the formation of an image at the periphery of a screen does not satisfactorily arrive at the image plane or ghosts are likely to occur. As the upper limit of 2.0 is exceeded, it is physically difficult to bend the optical path as in the case of conditions (1) and (2).
As can be understood from the foregoing, the air-based length, d, as defined by condition (3) should preferably be cut down by using as the optical path-bending element in the first lens group a prism in which entrance and exit surfaces are formed of planar surfaces or different in curvature from the lens surfaces on both sides of the first lens group and making the refractive index of a prism medium as high as possible. As the lower limit of 1.55 to condition (4) is not reached, it is physically difficult to bend the optical path. It is also preferable that nPRI does not exceed 1.90. Exceeding 1.90 means that the prism costs much, and renders ghosts likely to occur.
More preferably, at least one or all of the following conditions (1)xe2x80x2, (2)xe2x80x2, (3)xe2x80x2 and (4)xe2x80x2 should be satisfied.
1.5 less than xe2x88x92f11/{square root over ( )}(fWxc2x7fT) less than 2.2xe2x80x83xe2x80x83(1)xe2x80x2
1.3 less than f12/{square root over ( )}(fWxc2x7fT) less than 2.0xe2x80x83xe2x80x83(2)xe2x80x2
0.9 less than d/L less than 1.7xe2x80x83xe2x80x83(3)xe2x80x2
1.65 less than nPRIxe2x80x83xe2x80x83(4)xe2x80x2
Even more preferably, at least one of the following conditions (1)xe2x80x3, (2)xe2x80x3, (3)xe2x80x3 and (4)xe2x80x3 should be satisfied.
xe2x80x831.6 less than xe2x88x92f11/{square root over ( )}(fWxc2x7fT) less than 2.0xe2x80x83xe2x80x83(1)xe2x80x3
1.4 less than f12/{square root over ( )}(fWxc2x7fT) less than 1.8xe2x80x83xe2x80x83(2)xe2x80x3
1.0 less than d/L less than 1.5xe2x80x83xe2x80x83(3)xe2x80x3
1.75 less than nPRIxe2x80x83xe2x80x83(4)xe2x80x3
Most preferably, all conditions (1)xe2x80x3 to (4)xe2x80x3 should be satisfied.
Further, the zoom lens of the present invention should preferably satisfy the following condition (a).
1.8 less than fT/fWxe2x80x83xe2x80x83(a)
Here fW is the focal length of the zoom lens at the wide-angle end, and fT is the focal length of the zoom lens at the telephoto end.
Falling short of the lower limit of 1.8 to this condition means that the zoom ratio of the zoom lens becomes lower than 1.8. More preferably in this case, the value of fT/fW should not exceed 5.5. At greater than 5.5, the zoom ratio becomes high and the amount of the lens groups that move during zooming becomes too large. This causes the zoom lens to become large in the optical path-bending direction, and so renders it impossible to set up any compact imaging system.
Next, how to ensure any desired zoom ratio is explained. When the first lens group of the present invention has positive refracting power, the position of a principal point is evidently located nearer to the image side of the zoom lens as compared with the case where there is no optical path-bending reflecting optical element. This means that with the same refracting power, the position of an image point by the first lens group is located nearer to the image side; that is, an object point with respect to the second lens group is located at a farther position. Consequently, the magnification of the second lens group approaches zero, and the change in the focal length of the zoom lens becomes small even upon movement of the second lens group. One approach to solving this problem is to make the focal length of the first lens group short (so that the focal length of the zoom lens becomes short), whereby the focal length of the second lens group is increased to a certain degree to increase the magnification of the second lens group. According to the present invention wherein a combined system comprising the third lens group and the subsequent lens group (or groups) is also allowed to have a zooming function, if the magnifications, zoom ratios, etc. of both are artfully set, it is then possible to provide an efficient zooming of the zoom lens. Specific conditions to this end are determined by the following conditions (5), (6) and (7).
0.4 less than xe2x88x92xcex22W less than 1.2xe2x80x83xe2x80x83(5)
0.1 less than xe2x88x92xcex2RW less than 0.5xe2x80x83xe2x80x83(6)
0 less than log xcex3R/log xcex32 less than 1.3xe2x80x83xe2x80x83(7)
Here xcex22W is the magnification of the second lens group at the wide-angle end of the zoom lens upon focused on an infinite object point, xcex2RW is the composite magnification of a combined system comprising the third lens group and all subsequent lens groups at the wide-angle end upon focused on an infinite object point, xcex32 is xcex22T/xcex22W provided that xcex22T is the magnification of the second lens group at the telephoto end of the zoom lens upon focused on an infinite object point, and xcex3R is xcex2RT/xcex2RW provided that xcex2RT is the composite magnification of a combined system comprising the third lens group and all subsequent lens groups at the telephoto end upon focused on an infinite object point.
As the lower limits of 0.4 and 0.1 to conditions (5) and (6) are not reached, any satisfactorily high zoom ratio cannot be obtained throughout the zoom lens or the moving space becomes too large, resulting in a size increase. This in turn renders correction of various aberrations difficult, partly because the focal length of the first lens group becomes too short, and partly because the Petzval sum becomes large. Exceeding the upper limit of 1.3 to condition (7) is not preferable because fluctuations of the F-number and exit pupil position with zooming become too large. As the lower limit of 0 is not reached, the entrance pupil becomes too deep and the bending of the optical path becomes physically difficult. In any case, any satisfactorily high zoom ratio cannot be obtained throughout the zoom lens, or the moving space becomes too large, leading to a bulky size.
More preferably, at least one or all of the following conditions (5)xe2x80x2, (6)xe2x80x2 and (7)xe2x80x2 should be satisfied.
0.4 less than xe2x88x92xcex22W less than 1.1xe2x80x83xe2x80x83(5)xe2x80x2
0.20 less than xe2x88x92xcex2RW less than 0.45xe2x80x83xe2x80x83(6)xe2x80x2
0.15 less than log xcex3R/log xcex32 less than 1.2xe2x80x83xe2x80x83(7)xe2x80x2
Even more preferably, at least one of the following conditions (5)xe2x80x3, (6)xe2x80x3 and (7)xe2x80x3 should be satisfied.
0.6 less than xe2x88x92xcex22W less than 1.0xe2x80x83xe2x80x83(5)xe2x80x3
0.25 less than xe2x88x92xcex2RW less than 0.4xe2x80x83xe2x80x83(6)xe2x80x3
0.25 less than log xcex3R/log xcex32 less than 1.0xe2x80x83xe2x80x83(7)xe2x80x3
In order to satisfy conditions (5), (6) and (7), the following conditions (8) and (9), too, should preferably be satisfied.
1.6 less than f1/{square root over ( )}(fWxc2x7fT) less than 6.0xe2x80x83xe2x80x83(8)
1.1 less than xe2x88x92f2/{square root over ( )}(fWxc2x7fT) less than 2.2xe2x80x83xe2x80x83(9)
Here f1 is the focal length of the first lens group, f2 is the focal length of the second lens group, and fW and fT are the focal lengths of the zoom lens at the wide-angle and the telephoto end, respectively.
As the upper limit to condition (8) is exceeded, any satisfactorily high magnification cannot be obtained throughout the zoom lens or the moving space becomes too large, leading to a bulky size. As the lower limit is not reached, correction of off-axis aberrations and chromatic aberrations becomes difficult.
As the upper limit of 2.2 to condition (9) is exceeded, zooming efficiency becomes high thanks to an increase in the magnification of the second lens group; however, the efficiency may rather decrease because the amount of movement to obtain the same zoom ratio is proportional to the focal length. As the lower limit of 1.1 is not reached, the magnification of the second lens group comes close to zero, ending up with a zooming efficiency drop.
More preferably, the following conditions (8)xe2x80x2 and/or (9)xe2x80x2 should be satisfied.
1.9 less than f1/{square root over ( )}(fWxc2x7fT) less than 4.5xe2x80x83xe2x80x83(8)xe2x80x2
1.2 less than xe2x88x92f2/{square root over ( )}(fWxc2x7fT) less than 2.0xe2x80x83xe2x80x83(9)
Even more preferably, the following conditions (8)xe2x80x3 and/or (9)xe2x80x3 should be satisfied.
2.2 less than f1/{square root over ( )}(fWxc2x7fT) less than 3.0xe2x80x83xe2x80x83(8)xe2x80x3
1.3 less than xe2x88x92f2/{square root over ( )}(fWxc2x7fT) less than 1.8xe2x80x83xe2x80x83(9)xe2x80x3
Most preferably, both conditions (8)xe2x80x3 and (9)xe2x80x3 should be satisfied.
As the second lens group is designed with a high magnification, another problem arises. The magnification of the second lens group becoming high means that an object point with respect to a combined system that comprises the third lens group and the subsequent lens group or groups and has another zooming function is located at a farther position and the magnification of the combined system comes close to zero, resulting in a drop of zooming efficiency by that combined system. There are two approaches to solving this problem; one is to make the focal length of the combined system comprising the third lens group and the subsequent lens group or groups long to a certain degree, and another is to bring a principal point as close to an image point for the second lens group as possible. In the former case, the following condition (10) should preferably be satisfied.
0.8 less than fRW/{square root over ( )}(fWxc2x7fT) less than 1.7xe2x80x83xe2x80x83(10)
Here fRW is the composite focal length of the combined system comprising the third lens group and the subsequent lens group or groups, and fW and fT are the focal lengths of the zoom lens at the wide-angle and the telephoto end, respectively.
As the lower limit of 0.8 to condition (10) is not reached, the zooming efficiency by the combined system comprising the third lens group and the subsequent lens group or groups becomes worse. As the upper limit of 1.7 is exceeded, the zooming efficiency becomes worse for the same reason as in condition (9). In the latter case, the third lens group should preferably have therein at least one converging surface that is defined by an air contact surface convex on its object side and satisfies the following condition (b) and at least one diverging surface that is located on an image side with respect to the converging surface, is defined by an air contact surface convex on its image side and satisfies the following condition (b).
0 less than RP/fW less than 2xe2x80x83xe2x80x83(b)
0 less than RN/fW less than 4xe2x80x83xe2x80x83(c)
Here RP and RN are the axial radii of curvature of the converging surface and the diverging surface, respectively. Otherwise, it is difficult to bring the principal point for the third lens group close to the image point for the second lens group.
More preferably, the following condition (10)xe2x80x2 should be satisfied.
0.9 less than fRW/{square root over ( )}(fWxc2x7fT) less than 1.5xe2x80x83xe2x80x83(10)xe2x80x2
Most preferably, the following condition (10)xe2x80x3 should be satisfied.
1.0 less than fRW/{square root over ( )}(fWxc2x7fT) less than 1.3xe2x80x83xe2x80x83(10)xe2x80x3
Particularly preferably for both the cases, the focal length of the combined system comprising the third lens group and the subsequent lens group or groups should be increased upon zooming from the wide-angle end to the telephoto end, as defined by the following condition (11).
1.0 less than fRT/fRW less than 2.5xe2x80x83xe2x80x83(11)
Here fRW is the composite focal length of the combined system comprising the third lens group and all the subsequent lens groups at the wide-angle end, and fRT is the composite focal length of the combined system comprising the third lens group and all the subsequent lens groups at the telephoto end.
As the lower limit of 1.0 to condition (11) is not reached, the effect of the combined system comprising the third and subsequent lens groups on zooming becomes slender, the amount of movement of the second lens group increases and the entrance pupil becomes deep, and it is difficult to bend the optical path. As the upper limit of 2.5 is exceeded, fluctuations of F-number with zooming tend to become noticeable.
More preferably, the following condition (11)xe2x80x2 should be satisfied.
1.1 less than fRT/fRW less than 2.3xe2x80x83xe2x80x83(11)xe2x80x2
Most preferably, the following condition (11)xe2x80x3 should be satisfied.
1.2 less than fRT/fRW less than 2.1xe2x80x83xe2x80x83(11)xe2x80x3
According to the method most effective for the achievement of condition (11), the third lens group that, by definition, must be located as close to the image plane as possible at the wide-angle end with a view to obtaining high zoom ratios and the lens group located nearest to the object side of the zoom lens in the subsequent lens groups (hereinafter called the fourth lens group) should rather be located as near to the object side as possible, so that upon zooming to the telephoto side, the third lens group is moved toward the object side while the fourth lens group is moved toward the image side of the zoom lens (upon focusing on an infinite object point).
Specific conditions to this end are to satisfy the following conditions (12) and (13).
0.20 less than xe2x88x92M3/M2 less than 1.50xe2x80x83xe2x80x83(12)
0.15 less than xe2x88x92M4/M3 less than 1.00xe2x80x83xe2x80x83(13)
where M2 is the amount of movement of the second lens group from the wide-angle end to the telephoto end, M3 is the amount of movement of the third lens group from the wide-angle end to the telephoto end, and M4 is the amount of movement of the fourth lens group from the wide-angle end to the telephoto end, provided that the movement of each lens group toward the image side is of positive sign.
Exceeding the upper limit of 1.50 to condition (12) is not preferable because fluctuations of F-number and an exit pupil position with zooming become too noticeable. As the lower limit of 0.20 is not reached, the entrance pupil becomes too deep and so it is physically hard to bend the optical path. In any case, any satisfactorily high zoom ratio cannot be obtained throughout the zoom lens or the moving space becomes too large, leading to a bulky size.
As the upper limit of 1.00 to condition (13) is exceeded, the magnification of the combined system comprising the third and subsequent lens groups may become high. Since a main moving lens group is the fourth lens group for focusing, however, this is not preferable because fluctuations of magnification with focusing tend to become noticeable. As the lower limit of 0.15 is not reached, the principal point for the combined system comprising the third and subsequent lens groups is far away from the image point for the second lens group. This in turn causes a drop of zooming efficiency, or renders the focal length of the combined system comprising the third and subsequent lens group or groups likely to become long or the lens arrangement of the third and subsequent lens group or groups unreasonable, offering an obstacle to correction of aberrations.
More preferably, the following conditions (12)xe2x80x2 and/or (13)xe2x80x2 should be satisfied.
0.30 less than xe2x88x92M3/M2 less than 1.40xe2x80x83xe2x80x83(12)xe2x80x2
0.20 less than xe2x88x92M4/M3 less than 0.80xe2x80x83xe2x80x83(13)xe2x80x2
Even move preferably, the following condition (12)xe2x80x3 or (13)xe2x80x3 should be satisfied.
0.40 less than xe2x88x92M3/M2 less than 1.30xe2x80x83xe2x80x83(12)xe2x80x3
0.25 less than xe2x88x92M4/M3 less than 0.60xe2x80x83xe2x80x83(13)xe2x80x3
Most preferably, both conditions (12)xe2x80x3 and (13)xe2x80x3 should be satisfied.
It is noted that focusing should preferably be performed with the fourth lens group. It is then preferable to satisfy the following condition (14).
0.10 less than D34W/fW less than 0.70xe2x80x83xe2x80x83(14)
Here D34W is an air separation between the third lens group and the fourth lens group at the wide-angle end upon focused on an infinite object point, and fW is the focal length of the zoom lens at the wide-angle end.
As the lower limit of 0.10 to this condition is not reached, the third lens group is prone to interference with the fourth lens group for lack of any focusing space. As the upper limit of 0.70 is exceeded, conversely, the moving space for zooming tends to become insufficient.
More preferably,
0.15 less than D34W/fW less than 0.60xe2x80x83xe2x80x83(14)xe2x80x2
Most preferably,
0.20 less than D34W/fW less than 0.50xe2x80x83xe2x80x83(14)xe2x80x3
When focusing is performed by movement of the fourth lens group, on the other hand, astigmatism tends generally to be placed in an ill-balanced state. This astigmatism is likely to occur especially when residual astigmatism occurring at the 1st to 3rd lens groups is corrected at the fourth lens group. Thus, both refracting surfaces of any one of the lens components forming the third lens group, inclusive of the doublet component, should be configured as aspheric surfaces. It is also preferable to incorporate at least one doublet component of a positive and a negative lens element in the third lens group because chromatic aberrations should preferably be corrected at the third lens group that has generally high light rays. It is understood that the xe2x80x9clens componentxe2x80x9d used herein means a lens that contacts spaces on both sides alone and has any air contact surface nowhere on the optical path, e.g., a single lens or a doublet.
The construction of the third lens group is now explained in detail.
The third lens group may be made up of, in order from its object side:
1) a doublet component consisting of a positive lens element and a negative lens element and a single lens element configured as spherical surfaces at both surfaces, two subgroups or three lens elements in all,
2) a doublet component consisting of a single lens element configured as aspheric surfaces at both surfaces and a doublet component consisting of a positive lens element and a negative lens element, two subgroups or three lens elements in all, or
3) only a doublet component consisting of a positive lens element configured as aspheric surfaces at both air contact surfaces and a negative lens element, one group or two lens elements in all.
In any case, the doublet component may serve to slack the relative decentration sensitivity between the lens elements that form the third lens group.
Corresponding to the types 1), 2) and 3) of the third lens group, it is further preferable to satisfy the following conditions (15-1), (15-2) and (15-3), respectively (with respect to correction of aberrations and slacking of decentration sensitivity).
1.05 less than RC3/RC1 less than 3.00xe2x80x83xe2x80x83(15-1)
0.25 less than RC3/RC1 less than 0.75xe2x80x83xe2x80x83(15-2)
1.20 less than RC3/RC1 less than 3.60xe2x80x83xe2x80x83(15-3)
Here RC1 is the axial radius of curvature of the surface nearest to the object side of the doublet component, and RC3 is the axial radius of curvature of the surface nearest to the image side of the doublet component.
Exceeding the respective upper limits of 3.00, 0.75 and 3.60 to these conditions (15-1), (15-2) and (15-3) may be favorable for correction of spherical aberrations, coma and astigmatism throughout the zoom lens; however, the effect of cementing on slacking of decentration sensitivity becomes slender. As the respective lower limits of 1.05, 0.25 and 1.20 are not reached, correction of spherical aberrations, coma and astigmatism throughout the zoom lens becomes difficult.
More preferably,
1.15 less than RC3/RC1 less than 2.50xe2x80x83xe2x80x83(15-1)xe2x80x2
0.30 less than RC3/RC1 less than 0.65xe2x80x83xe2x80x83(15-2)xe2x80x2
1.40 less than RC3/RC1 less than 3.00xe2x80x83xe2x80x83(15-3)xe2x80x2
Most preferably,
1.25 less than RC3/RC1 less than 2.00xe2x80x83xe2x80x83(15-1)xe2x80x3
0.35 less than RC3/RC1 less than 0.55xe2x80x83xe2x80x83(15-2)xe2x80x3
1.60 less than RC3/RC1 less than 2.40xe2x80x83xe2x80x83(15-3)xe2x80x3
Furthermore corresponding to the types 1), 2) and 3) of the third lens group, it is preferable to satisfy the following conditions (16-1) and (17-1), (16-2) and (17-2), and (16-3) and (17-3) with respect to correction of chromatic aberrations.
xe2x88x920.7 less than L/RC2 less than 0.1xe2x80x83xe2x80x83(16-1)
10 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-1)
xe2x80x83xe2x88x920.5 less than L/RC2 less than 0.3xe2x80x83xe2x80x83(16-2)
20 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-2)
xe2x88x920.9 less than L/RC2 less than xe2x88x920.1xe2x80x83xe2x80x83(16-3)
10 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-3)
Here L is the diagonal length in mm of an effective image pickup area of the electronic image pickup device, RC2 is the axial radius of curvature of a cementing surface of the doublet component in the third lens group, xcexdCP is the d-line based Abbe number of a medium of the positive lens element of the doublet component in the third lens group, and xcexdCN is the d-line based Abbe number of a medium of the negative lens element of the doublet component in the third lens group with the proviso that the electronic image pickup device is used in such a way as to include an angle of view of 55xc2x0 or greater at the wide-angle end.
Falling short of the respective lower limits of xe2x88x920.7, xe2x88x920.5 and xe2x88x920.9 to conditions (16-1), (16-2) and (16-3) may be favorable for correction of longitudinal chromatic aberration and chromatic aberration of magnification; however, this is not preferable because chromatic aberration of spherical aberration is likely to occur, and spherical aberrations at short wavelengths remain over-corrected even when spherical aberrations at the reference wavelength can be well corrected, causing chromatic blurring of images. As the respective upper limits of 0.1, 0.3 and xe2x88x920.1 are exceeded, correction of longitudinal chromatic aberration and chromatic aberration of magnification tends to become insufficient and spherical aberrations at short wavelengths are prone to under-correction.
As the respective lower limits of 10, 20 and 10 to conditions (17-1), (17-2) and (17-3) are not reached, correction of longitudinal chromatic aberration tends to become insufficient. The upper limits to conditions (17-1), (17-2) and (17-3) may prima facie be set at 90. Any combinations of media exceeding the upper limit of 90 do not occur in nature. A preferable upper limit to xcexdCPxe2x88x92xcexdCN is 60. Materials of greater than 60 are expensive.
More preferably, either one or both of the following conditions (16-1)xe2x80x2 and (17-1)xe2x80x2, (16-2)xe2x80x2 and (17-2)xe2x80x2, and (16-3)xe2x80x2 and (17-3)xe2x80x2 should be satisfied.
xe2x88x920.6 less than L/RC2 less than 0.0xe2x80x83xe2x80x83(16-1)xe2x80x2
15 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-1)xe2x80x2
xe2x88x920.4 less than L/RC2 less than 0.2xe2x80x83xe2x80x83(16-2)xe2x80x2
25 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-2)xe2x80x2
xe2x88x920.8 less than L/RC2 less than xe2x88x920.2xe2x80x83xe2x80x83(16-3)xe2x80x2
15 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-3)xe2x80x2
Even more preferably, either one of the following conditions (16-1)xe2x80x3 and (17-1)xe2x80x3, (16-2)xe2x80x3 and (17-2)xe2x80x3, and (16-3)xe2x80x3 and (17-3)xe2x80x3 should be satisfied.
xe2x88x920.5 less than L/RC2 less than xe2x88x920.1xe2x80x83xe2x80x83(16-1)xe2x80x3
20 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-1)xe2x80x3
xe2x80x83xe2x88x920.3 less than L/RC2 less than 0.1xe2x80x83xe2x80x83(16-2)xe2x80x3
30 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-2)xe2x80x3
xe2x88x920.7 less than L/RC2 less than xe2x88x920.3xe2x80x83xe2x80x83(16-3)xe2x80x3
20 less than xcexdCPxe2x88x92xcexdCNxe2x80x83xe2x80x83(17-3)xe2x80x3
Most preferably, both of the above conditions (16-1)xe2x80x3 and (17-1)xe2x80x3, (16-2)xe2x80x3 and (17-2)xe2x80x3, and (16-3)xe2x80x3 and (17-3)xe2x80x3 should be satisfied.
The fourth lens group should preferably be composed of one positive lens component and satisfy the following conditions (18) and (19).
xe2x88x924.00 less than (R4F+R4R)/(R4Fxe2x88x92R4R) less than 0.0xe2x80x83xe2x80x83(18)
0.10 less than L/f4 less than 0.70xe2x80x83xe2x80x83(19)
Here R4F is the axial radius of curvature of the object side-surface of the positive lens component, R4R is the axial radius of curvature of the image side-surface of the positive lens component, L is the diagonal length of an effective image pickup area of the electronic image pickup device, and f4 is the focal length of the fourth lens group.
Exceeding the upper limit of 0.0 to condition (18) is not preferable for zooming efficiency because a principal point for the combined system comprising the third and subsequent lens groups tends to be far away from the image point by the second lens group. As the lower limit of xe2x88x924.00 is not reached, fluctuations of astigmatism with focusing tend to become large.
As the upper limit of 0.70 to condition (19) is exceeded, the third and fourth lens groups cannot move in opposite directions during zooming. Falling short of the lower limit of 0.10 is not preferable because the amount of movement of the fourth lens group for focusing becomes too large.
More preferably, either one or both of the following conditions (18)xe2x80x2 and (19)xe2x80x2 should be satisfied.
xe2x88x923.60 less than (R4F+R4R)/(R4Fxe2x88x92R4R) less than xe2x88x920.4xe2x80x83xe2x80x83(18)xe2x80x2
0.15 less than L/f4 less than 0.60xe2x80x83xe2x80x83(19)xe2x80x2
Even more preferably, either one of the following conditions (18)xe2x80x3 and (19)xe2x80x3 should be satisfied.
xe2x88x923.20 less than (R4F+R4R)/(R4Fxe2x88x92R4R) less than xe2x88x920.80xe2x80x83xe2x80x83(18)xe2x80x2
0.20 less than L/f4 less than 0.50xe2x80x83xe2x80x83(19)xe2x80x2
Most preferably, both of the above conditions (18)xe2x80x3 and (19)xe2x80x3 should be satisfied.
For the second lens group having a long focal length, it should be only composed of, in order from its object side, a negative lens element and a positive lens element, two lens elements in all. In conjunction with the first lens group, it is preferable to satisfy the following conditions (20) and (21).
xe2x88x920.80 less than (R1PF+R1PR)/(R1PFxe2x88x92R1PR) less than 0.90xe2x80x83xe2x80x83(20)
xe2x88x920.10 less than (R2NF+R2NR)/(R2NFxe2x88x92R2NR) less than 2.00xe2x80x83xe2x80x83(21)
Here R1PF is the axial radius of curvature of the object side-surface of the positive lens component in the first lens group, R1PR is the axial radius of curvature of the image side-surface of the positive component in the first lens group, R2NF is the axial radius of curvature of the object side-surface of the negative lens component in the second lens group, and R2NR is the axial radius of curvature of the image side-surface of the negative lens component in the second lens group.
As the upper limit of 0.90 to condition (20) is exceeded, higher-order chromatic aberrations of magnification tend to occur, and as the lower limit of xe2x88x920.80 is not reached, the entrance pupil tends to become deep.
As the upper limit of 2.00 to condition (20) is exceeded, coma tends to occur, and as the lower limit of xe2x88x920.10 is not reached, barrel distortion tends to occur.
More preferably, either one or both of the following conditions (20)xe2x80x2 and (21)xe2x80x2 should be satisfied.
xe2x88x920.50 less than (R1PF+R1PR)/(R1PFxe2x88x92R1PR) less than 0.70xe2x80x83xe2x80x83(20)xe2x80x2
xe2x88x920.20 less than (R2NF+R2NR)/(R2NFxe2x88x92R2NR) less than 1.50xe2x80x83xe2x80x83(21)xe2x80x2
Even more preferably, either one of the following conditions (20)xe2x80x3 and (21)xe2x80x3 should be satisfied.
xe2x88x920.20 less than (R1PF+R1PF)/(R1PFxe2x88x92R1PR less than 0.50xe2x80x83xe2x80x83(20)xe2x80x3
0.50 less than (R2NF+R2NR)/(R2NFxe2x88x92R2NR) less than 1.50xe2x80x83xe2x80x83(21)xe2x80x3
Most preferably, both of the above conditions (20)xe2x80x3 and (21)xe2x80x3 should be satisfied.
The presumption for the electronic image pickup device used herein is that it has a total angle of view of 55xc2x0 or greater at the wide-angle end. The 55 degrees are the wide-angle-end total angle of view needed commonly for electronic image pickup devices.
For the electronic image pickup device, the wide-angle-end total angle of view should preferably be 80xc2x0 or smaller. At greater than 80xc2x0, distortions are likely to occur, and it is difficult to make the first lens group compact. It is thus difficult to slim down the electronic imaging system.
Thus, the present invention provides means for reducing the thickness of the zoom lens portion while satisfactory image-formation capability is maintained.
Next, how and why the thickness of filters is reduced is now explained. In an electronic imaging system, an infrared absorption filter having a certain thickness is usually inserted between an image pickup device and the object side of a zoom lens, so that the incidence of infrared light on the image pickup plane is prevented. Here consider the case where this filter is replaced by a coating devoid of thickness. In addition to the fact that the system becomes thin as a matter of course, there are spillover effects. When a near-infrared sharp cut coat having a transmittance (xcfx84600) of at least 80% at 600 nm and a transmittance (xcfx84700) of up to 8% at 700 nm is introduced between the image pickup device in the rear of the zoom lens system and the object side of the system, the transmittance at a near-infrared area of 700 nm or longer is relatively lower and the transmittance on the red side is relatively higher as compared with those of the absorption type, so that the tendency of bluish purple to turn into magentaxe2x80x94a defect of a CCD or other solid-state image pickup device having a complementary colors filterxe2x80x94is diminished by gain control and there can be obtained color reproduction comparable to that by a CCD or other solid-state image pickup device having a primary colors filter. In addition, it is possible to improve on color reproduction of, to say nothing of primary colors and complementary colors, objects having strong reflectivity in the near-infrared range, like plants or the human skin.
Thus, it is preferable to satisfy the following conditions (22) and (23):
xcfx84700/xcfx84550xe2x89xa70.8xe2x80x83xe2x80x83(22)
xcfx84700/xcfx84550xe2x89xa60.08xe2x80x83xe2x80x83(23)
where xcfx84550 is the transmittance at 550 nm wavelength.
More preferably, the following conditions (22)xe2x80x2 and/or (23)xe2x80x2 should be satisfied:
xcfx84600/xcfx84550xe2x89xa70.85xe2x80x83xe2x80x83(22)
xcfx84700/xcfx84550xe2x89xa60.05xe2x80x83xe2x80x83(23)
Even more preferably, the following conditions (22)xe2x80x3 or (23)xe2x80x3 should be satisfied:
xcfx84600/xcfx84550xe2x89xa70.9xe2x80x83xe2x80x83(22)xe2x80x3
xcfx84700/xcfx84550xe2x89xa60.03xe2x80x83xe2x80x83(23)xe2x80x3
Most preferably, both conditions (28)xe2x80x3 and (29)xe2x80x2 should be satisfied.
Another defect of the CCD or other solid-state image pickup device is that the sensitivity to the wavelength of 550 nm in the near ultraviolet range is considerably higher than that of the human eye. This, too, makes noticeable chromatic blurring at the edges of an image due to chromatic aberrations in the near-ultraviolet range. Such color blurring is fatal to a compact optical system. Accordingly, if an absorber or reflector is inserted on the optical path, which is designed such that the ratio of the transmittance (xcfx84400) at 400 nm wavelength to that (xcfx84550) at 550 nm wavelength is less than 0.08 and the ratio of the transmittance (xcfx84440) at 440 nm wavelength to that (xcfx84550) at 550 nm wavelength is greater than 0.4, it is then possible to considerably reduce noises such as chromatic blurring while the wavelength range necessary for color reproduction (satisfactory color reproduction) is kept intact.
It is thus preferable to satisfy the following conditions (24) and (25):
xcfx84400/xcfx84550xe2x89xa60.08xe2x80x83xe2x80x83(24)
xcfx84440/xcfx84550xe2x89xa70.4xe2x80x83xe2x80x83(25)
More preferably, the following conditions (24)xe2x80x2 and/or (25)xe2x80x2 should be satisfied.
xcfx84400/xcfx84550xe2x89xa60.06xe2x80x83xe2x80x83(24)xe2x80x2
xcfx84440/xcfx84550xe2x89xa70.5xe2x80x83xe2x80x83(25)xe2x80x2
Even more preferably, the following condition (24)xe2x80x3 or (25)xe2x80x3 should be satisfied.
xcfx84400/xcfx84550xe2x89xa60.04xe2x80x83xe2x80x83(24)xe2x80x3
xcfx84440/xcfx84550xe2x89xa70.6xe2x80x83xe2x80x83(25)xe2x80x3
Most preferably, both condition (24)xe2x80x3 and (25)xe2x80x3 should be satisfied.
It is noted that these filters should preferably be located between the image-formation optical system and the image pickup device.
On the other hand, a complementary colors filter is higher in substantial sensitivity and more favorable in resolution than a primary colors filter-inserted CCD due to its high transmitted light energy, and provides a great merit when used in combination with a small-size CCD.
To shorten and slim down the optical system, the optical low-pass filter that is another filter, too, should preferably be thinned as much as possible. In general, an optical low-pass filter harnesses a double-refraction action that a uniaxial crystal like berg crystal has. However, when the optical low-pass filter includes a quartz optical low-pass filter or filters in which the angles of the crystal axes with respect to the optical axis of the zoom lens are in the range of 35xc2x0 to 55xc2x0 and the crystal axes are in varying directions upon projected onto the image plane, the filter having the largest thickness along the optical axis of the zoom lens among them should preferably satisfy the following condition (26) with respect to its thickness tLPF (mm).
0.08 less than tLPF/a less than 0.16 (at a less than 4 xcexcm)
0.075 less than tLPF/a less than 0.15 (at a less than 3 xcexcm)xe2x80x83xe2x80x83(26)
Here tLPF (mm) is the thickness of the optical low-pass filter having the largest thickness along the optical axis of the zoom lens with the angle of one crystal axis with respect to the optical axis being in the range of 35xc2x0 to 55xc2x0, and a is the horizontal pixel pitch (in xcexcm) of the image pickup device.
Referring to a certain optical low-pass filter or an optical low-pass filter having the largest thickness among optical low-pass filters, its thickness is set in such a way that contrast becomes theoretically zero at the Nyquist threshold wavelength, i.e., at approximately a/5.88 (mm). A thicker optical low-pass filter may be effective for prevention of swindle signals such as moire fringes, but makes it impossible to take full advantages of the resolving power that the electronic image pickup device has, while a thinner filter renders full removal of swindle signals like moire fringes impossible. However, swindle signals like moire fringes have close correlations with the image-formation capability of a taking lens like a zoom lens; high image-formation capability renders swindle signals like moire fringes likely to occur. Accordingly, when the image-formation capability is high, the optical low-pass filter should preferably be somewhat thicker whereas when it is low, the optical low-pass filter should preferably be somewhat thinner.
As the pixel pitch becomes small, on the other hand, the contrast of frequency components greater than the Nyquist threshold decreases due to the influence of diffraction by the image-formation lens system and, hence, swindle signals like moire fringes are reduced. Thus, it is preferable to reduce the thickness of the optical low-pass filter by a few % or a few tens % from a/5.88 (mm) because a rather improved contrast is obtainable at a spatial frequency lower than the frequency corresponding to the Nyquist threshold.
More preferably,
0.075 less than tLPF/a less than 0.15 (at a less than 4 xcexcm)
0.07 less than tLPF/a less than 0.14 (at a less than 3 xcexcm)xe2x80x83xe2x80x83(26)xe2x80x2
Most preferably,
0.07 less than tLPF/a less than 0.14 (at a less than 4 xcexcm)
0.065 less than tLPF/a less than 0.13 (at a less than 3 xcexcm)xe2x80x83xe2x80x8326)xe2x80x3
If an optical low-pass filter is too thin at a less than 4 xcexcm, it is then difficult to process. Thus, it is permissible to impart some thickness to the optical low-pass filter or make high the spatial frequency (cutoff frequency) where contrast reduces down to zero even when the upper limit to conditions (26), (26)xe2x80x2 and (26)xe2x80x3 is exceeded. In other words, it is permissible to regulate the angle of the crystal axis of the optical low-pass filter with respect to the optical axis of the zoom lens to within the range of 15xc2x0 to 35xc2x0 or 55xc2x0 to 75xc2x0. In some cases, it is also permissible to dispense with the optical low-pass filter. In that angle range, the quantity of separation of incident light to an ordinary ray and an extraordinary ray is smaller than that around 45xc2x0, and that separation does not occur at 0xc2x0 or 90xc2x0 (at 90xc2x0, however, there is a phase difference because of a velocity difference between both raysxe2x80x94the quarter-wave principle).
As already described, when the pixel pitch becomes small, it is difficult to increase the F-number because the image-formation capability deteriorates under the influence of diffraction at a high spatial frequency that compensates for such a small pixel pitch. It is thus acceptable to use two types of aperture stops for a camera, i.e., a full-aperture stop where there is a considerable deterioration due to geometric aberrations and an aperture stop having an F-number in the vicinity of diffraction limited. It is then acceptable to dispense with such an optical low-pass filter as described before.
Especially when the pixel pitch is small and the highest image-formation capability is obtained at a full-aperture stop, etc., it is acceptable to use an aperture stop having a constantly fixed inside diameter as means for controlling the size of an incident light beam on the image pickup plane instead of using an aperture stop having a variable inside diameter or a replaceable aperture stop. Preferably in that case, at least one of lens surfaces adjacent to the aperture stop should be set such that its convex surface is directed to the aperture stop and it extends through the inside diameter portion of the aperture stop, because there is no need of providing any additional space for the stop, contributing to length reductions of the zoom optical system. It is also desirable to locate an optical element having a transmittance of up to 90% (where possible, the entrance and exit surfaces of the optical element should be defined by planar surfaces) in a space including the optical axis at least one lens away from the aperture stop or use means for replacing that optical element by another element having a different transmittance.
Alternatively, the electronic imaging system is designed in such a way as to have a plurality of apertures each of fixed aperture size, one of which can be inserted into any one of optical paths between the lens surface located nearest to the image side of the first lens group and the lens surface located nearest to the object side of the third lens group and can be replaced with another as well, so that illuminance on the image plane can be adjusted. Then, media whose transmittances with respect to 550 nm are different but less than 80% are filled in some of the plurality of apertures for light quantity control. Alternatively, when control is carried out in such a way as to provide a light quantity corresponding to such an F-number as given by a (xcexcm)/F-number less than 4.0, it is preferable to fill the apertures with medium whose transmittance with respect to 550 nm are different but less than 80%. In the range of the full-aperture value to values deviating from the aforesaid condition as an example, any medium is not used or dummy media having a transmittance of at least 91% with respect to 550 nm are used. In the range of the aforesaid condition, it is preferable to control the quantity of light with an ND filter or the like, rather than to decrease the diameter of the aperture stop to such an extent that the influence of diffraction appears.
Alternatively, it is acceptable to uniformly reduce the diameters of a plurality of apertures inversely with the F-numbers, so that optical low-pass filters having different frequency characteristics can be inserted in place of ND filters. As degradation by diffraction becomes worse with stop-down, it is desirable that the smaller the aperture diameter, the higher the frequency characteristics the optical low-pass filters have.
It is understood that when the relation of the full-aperture F-number at the wide-angle end to the pixel pitch a (xcexcm) used satisfies F greater than a, it is acceptable to dispense with the optical low-pass filter. In other words, it is permissible that the all the medium on the optical axis between the zoom lens system and the electronic image pickup device is composed of air or a non-crystalline medium alone. This is because there are little frequency components capable of producing distortions upon bending due to a deterioration in the image-formation capability by reason of diffraction and geometric aberrations.
It is noted that satisfactory zoom lenses or electronic imaging systems may be set up by suitable combinations of the above conditions and the arrangements of the zoom lens and the electronic imaging system using the same.
It is understood that only the upper limit or only the lower limit may be applied to each of the above conditions, and that the values of these conditions in each of the following examples may be extended as far as the upper or lower limits thereof.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts that will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.